1. Field of the Invention
The present invention relates to an abnormality diagnosis device of an internal combustion engine having sensors for sensing an air-fuel ratio or a rich/lean state of exhaust gas, the sensors being arranged respectively upstream and downstream of a catalyst, which is used for purifying exhaust gas.
2. Description of Related Art
Aiming to improve an exhaust gas purification rate, there is a control system of an internal combustion engine mounted in a recent vehicle that has sensors (air-fuel ratio sensors or oxygen sensors) for sensing an air-fuel ratio or a rich/lean state of exhaust gas such that the sensors are arranged respectively upstream and downstream of a catalyst, which is used for purifying exhaust gas. The control system performs main feedback control for performing feedback correction of a supply air-fuel ratio (e.g., a fuel injection quantity) to conform the air-fuel ratio of the exhaust gas upstream of the catalyst to an upstream target air-fuel ratio based on an output of the upstream sensor. In addition, the control system performs sub feedback control for performing feedback correction of a control amount related to the main feedback control (for example, the upstream target air-fuel ratio or a main feedback correction amount) to conform the air-fuel ratio of the exhaust gas downstream of the catalyst to a downstream target air-fuel ratio based on an output of the downstream sensor.
Patent document 1 (Japanese Patent Gazette No. 3438298) describes an abnormality diagnosis of a system that performs the main feedback control and the sub feedback control. In the abnormality diagnosis described in Patent document 1, at least one of time in which output voltage of the downstream sensor is equal to or higher than a predetermined upper limit and time in which the output voltage is equal to or lower than a predetermined lower limit is integrated. When the integrated time does not reach a predetermined value within a specified time, it is determined that output fluctuation of the downstream sensor has become equal to or lower than a predetermined value, and it is determined that the downstream sensor is abnormal.
There is a possibility that variation (imbalance) is caused in the air-fuel ratio among cylinders of the internal combustion engine due to an error of fuel injection quantity of the cylinder, an error of air changing quantity of the cylinder and the like. If the air-fuel ratio variation among the cylinders increases in the system that performs the main feedback control and the sub feedback control as described above, fluctuation of the air-fuel ratio of the exhaust gas passing through the upstream sensor increases and deviation of the air-fuel ratio sensed with the upstream sensor from an average air-fuel ratio of all the cylinders increases Therefore, in some cases, a control center of the main feedback control shifts and the air-fuel ratio of the exhaust gas flowing into the catalyst goes out of a purification window of the catalyst. As a result, there is a possibility that an exhaust gas purification rate of the catalyst deteriorates and the exhaust emission deteriorates.
In recent years, it has been increasingly required to detect an abnormal state, in which the control center of the main feedback control shifts due to the air-fuel ratio variation among the cylinders and the exhaust gas purification rate deteriorates, in an early stage. The technology described in Patent document 1 detects the abnormality of the downstream sensor in the system that performs the main feedback control and the sub feedback control. However, the technology cannot detect an abnormal state where the exhaust gas purification rate is deteriorated by the air-fuel ratio variation among the cylinders (i.e., the state where the control center of the main feedback control has shifted).